A conventional bicycle includes a chain gear between its front wheel and a rear wheel in order to allow a user to manually change gear ratios (or simply “change gears”). However, some circumstances render it very difficult for a user to change gears when a road condition changes so rapidly, and to maintain a proper speed even when the user succeeds in changing gears. An electrical bicycle may overcome such problems to some extent. However, the electrical bicycle requires expensive sensors to measure an rpm of a wheel, to monitor a slope of a road, and the like, and may also suffer from frequent malfunctions of such sensors, not to mention their high cost.